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POWhy is 'box' instruction emitted for generic?
 

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  1. POWhy is 'box' instruction emitted for generic?
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    <p>Here is fairly simple generic class. Generic parameter is constrained to be reference type. <code>IRepository</code> and <code>DbSet</code> also contain the same constraint.</p> <pre><code>public class Repository&lt;TEntity&gt; : IRepository&lt;TEntity&gt; where TEntity : class, IEntity { protected readonly DbSet&lt;TEntity&gt; _dbSet; public void Insert(TEntity entity) { if (entity == null) throw new ArgumentNullException("entity", "Cannot add null entity."); _dbSet.Add(entity); } } </code></pre> <p>Compiled IL contains <code>box</code> instruction. Here is the release version (debug version also contains it though).</p> <pre><code>.method public hidebysig newslot virtual final instance void Insert(!TEntity entity) cil managed { // Code size 38 (0x26) .maxstack 8 IL_0000: ldarg.1 &gt;&gt;&gt;IL_0001: box !TEntity IL_0006: brtrue.s IL_0018 IL_0008: ldstr "entity" IL_000d: ldstr "Cannot add null entity." IL_0012: newobj instance void [mscorlib]System.ArgumentNullException::.ctor(string, string) IL_0017: throw IL_0018: ldarg.0 IL_0019: ldfld class [EntityFramework]System.Data.Entity.DbSet`1&lt;!0&gt; class Repository`1&lt;!TEntity&gt;::_dbSet IL_001e: ldarg.1 IL_001f: callvirt instance !0 class [EntityFramework]System.Data.Entity.DbSet`1&lt;!TEntity&gt;::Add(!0) IL_0024: pop IL_0025: ret } // end of method Repository`1::Insert </code></pre> <p><strong>UPDATE:</strong> </p> <p>With <code>object.Equals(entity, default(TEntity))</code> it looks even worse:</p> <pre><code> .maxstack 2 .locals init ([0] !TEntity CS$0$0000) IL_0000: ldarg.1 &gt;&gt;&gt;IL_0001: box !TEntity IL_0006: ldloca.s CS$0$0000 IL_0008: initobj !TEntity IL_000e: ldloc.0 &gt;&gt;&gt;IL_000f: box !TEntity IL_0014: call bool [mscorlib]System.Object::Equals(object, object) IL_0019: brfalse.s IL_002b </code></pre> <p><strong>UPDATE2:</strong></p> <p>For those who are interested, here is the code compiled by jit shown in debugger:</p> <pre><code>0cd5af28 55 push ebp 0cd5af29 8bec mov ebp,esp 0cd5af2b 83ec18 sub esp,18h 0cd5af2e 33c0 xor eax,eax 0cd5af30 8945f0 mov dword ptr [ebp-10h],eax 0cd5af33 8945ec mov dword ptr [ebp-14h],eax 0cd5af36 8945e8 mov dword ptr [ebp-18h],eax 0cd5af39 894df8 mov dword ptr [ebp-8],ecx //entity reference to [ebp-0Ch] 0cd5af3c 8955f4 mov dword ptr [ebp-0Ch],edx //some debugger checks 0cd5af3f 833d9424760300 cmp dword ptr ds:[3762494h],0 0cd5af46 7405 je 0cd5af4d Branch 0cd5af48 e8e1cac25a call clr!JIT_DbgIsJustMyCode (67987a2e) 0cd5af4d c745fc00000000 mov dword ptr [ebp-4],0 0cd5af54 90 nop //comparison or entity ref with zero 0cd5af55 837df400 cmp dword ptr [ebp-0Ch],0 0cd5af59 0f95c0 setne al 0cd5af5c 0fb6c0 movzx eax,al 0cd5af5f 8945fc mov dword ptr [ebp-4],eax 0cd5af62 837dfc00 cmp dword ptr [ebp-4],0 //if not zero, jump further 0cd5af66 7542 jne 0cd5afaa Branch //throwing exception here </code></pre> <p>The reason of this question is actually that NDepend warns about using boxing/unboxing. I was curious why it found boxing in some generic classes, and now it's clear.</p>
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    1. COIs it the same if you use object.Equals(entity, default(TEntity))?
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      1. USKonrad Kokosa
    2. COI'm gonna take a stab in the dark here. IIRC, interfaces _can_ cause boxing of value types, so I wonder if that has something to do with it. Perhaps the compiler thinks it must box it in case a value-type is fed in for it to check against the `null` reference. I might be full of it though. :) EDIT: this is despite the constraint that it's a `class` (not a value type) but I'm not certain whether or not that's considered by the compiler and/or CLR and/or IL.
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      1. USChris Sinclair
    3. COI would also suspect, that both `object.Equals` and `==` operator is so _general_ that it boxes by default. Try `EqualityComparer<TEntity>.Default.Equals(entity, default(T))` as it should use generic method.
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